Two main methods for determining the time or phase position of a signal pulse train are known to the art. These methods are known respectively as the zero-crossing timing method and the complex vector method.
When practising the zero-crossing method, the desired time information is obtained by registering the position in a reference-divider chain at those moments when the pulse train changes sign.
The time resolution obtained with this method is quite simply one period of the highest reference frequency signal produced on the input of the reference frequency-divider chain. For example, in order to determine the phase position of a pulse train having a repetition frequency of 1 MHZ with a resolution of 1 degree, it would be necessary to apply a frequency of 360 MHZ to the input of the frequency-divider chain. Consequently, the logic used to determine the phase would need to be very fast, even in the case of low-frequency pulse trains of moderate resolution.
When practising the complex vector method, the information desired is assumed to be included in the fundamental sine component of the pulse train. This sine component is filtered-out and resolved into two quadrature components, by correlation with sine and cosine reference-frequencies in balanced mixers. Consequently, it is then necessary to digitize the two results and to process the arc-tangent of their ratio in a computer in order to determine the phase.